One type of RFID (Radio Frequency Identification) devices are half-duplex RFID transponders with an integrated three-dimensional front-end circuit. These RFID transponder comprise three LC resonant circuits arranged in a three-dimensional configuration and each LC resonant circuit is coupled to a storage capacitor which is charged during a capacitor charging phase by energy comprised in an RF signal which is received by the three LC resonant circuits.
RFID systems including RFID transponders and an interrogator unit are used for example in portable identification devices such as passive entry and immobilizer keys for vehicles. In this case, the interrogator unit is usually placed in the vehicle and the transponder is carried by the driver in form of a tag or a chip card. Typically, these RFID systems operate at a frequency in a low frequency (LF) range around 125 kilohertz or 134 kilohertz.
Active transponders are battery powered, whereas passive transponders have no autonomous power supply. Instead, they use RF energy received with an LC resonant circuit from the interrogator unit during an interrogation interval by rectifying the received RF signal and charging a storage capacitor with the rectified signal. Combined systems are known where a battery is provided as a backup solution, in case that the charged power is insufficient.
Passive transponders are usually realized as half-duplex (HDX) transponders. A HDX transponder receives in a first time an interrogation RF signal. The end of the interrogation interval is detected by an end-of-burst (EOB) detector. The interrogation interval is followed by a response interval during which the transponder is expected to send some response, e.g. an ID code or some other data. Energy for operating the transponder when sending the response during the response interval is supplied by the storage capacitor.
Transponders with only one antenna are sensitive to orientation. Therefore, advanced transponders are provided with three antennas in the form of three LC resonant circuits which are arranged in a three-dimensional configuration. The three antenna circuits have antenna structures that are physically oriented at mutually 90 degrees. With such a transponder, signals from a transceiver/interrogator placed for example in a vehicle are detected independently of orientation in space of the transponder.
While it is an advantageous to have three LC resonant circuits, this means that three receiver channels are needed. On the other hand, it is important that power consumption of the transponder during charging the storage capacitor must be as low as possible.
Each of the three antennas is directed in a different direction in order to provide optimum reception regardless of the orientation and location of the device. Since a radio frequency signal received with the antenna may have varying field strength dependent on the specific orientation, it is necessary to determine the channel or in other words the antenna having the highest field strength of the received signal. This is usually performed by a channel selector.
The RFID system with three antennas in each spatial direction should ensure proper functionality even if the RFID device is displaced or rotated with respect to the reader. In order to ensure that the RFID tag sends over the antenna with the best coupling to the reader, the strongest channel during downlink is to be detected.